Datasheet

ADC122S706

SRC

+ V

REF

- V

REF

ADC122S706

SRC

REF

ADC122S706

SNAS408A –NOVEMBER 2007–REVISED MARCH 2013

www.ti.com

ANALOG SIGNAL INPUTS

The ADC122S706 has dual differential inputs where the effective input voltage that is digitized is CHA+ minus

CHA− (DIFFINA) and CHB+ minus CHB− (DIFFINB). As is the case with all differential input A/D converters,

operation with a fully differential input signal or voltage will provide better performance than with a single-ended

input. However, the ADC122S706 can be presented with a single-ended input.

The current required to recharge the input sampling capacitor will cause voltage spikes at the + and − inputs. Do

not try to filter out these noise spikes. Rather, ensure that the transient settles out during the acquisition period

(three SCLK cycles after the fall of CS).

Differential Input Operation

With a fully differential input voltage or signal, a positive full scale output code (0111 1111 1111b or 7FFh) will be

obtained when DIFFINA or DIFFINB is greater than or equal to V

REF

− 1.5 LSB. A negative full scale code (1000

0000 0000b or 800h) will be obtained when DIFFINA or DIFFINB is greater than or equal to −V

REF

+ 0.5 LSB.

This ignores gain, offset and linearity errors, which will affect the exact differential input voltage that will

determine any given output code. Figure 44 shows the ADC122S706 being driven by a full-scale differential

source.

Figure 44. Differential Input

Single-Ended Input Operation

For single-ended operation, the non-inverting inputs of the ADC122S706 can be driven with a signal that has a

maximum to minimum value range that is equal to or less than twice the reference voltage. The inverting inputs

should be biased at a stable voltage that is halfway between these maximum and minimum values. In order to

utilize the entire dynamic range of the ADC122S706, the reference voltage is limited at V

/ 2. This allows the

non-inverting inputs the maximum swing range of ground to V

. Figure 45 shows the ADC122S706 being driven

by a full-scale single-ended source.

Figure 45. Single-Ended Input

Since the design of the ADC122S706 is optimized for a differential input, the performance degrades slightly when

driven with a single-ended input. Linearity characteristics such as INL and DNL typically degrade by 0.1 LSB and

dynamic characteristics such as SINAD typically degrades by 2 dB. Note that single-ended operation should only

be used if the performance degradation (compared with differential operation) is acceptable.

Product Folder Links: ADC122S706